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Problem with multi-SF predecessors

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Nader K. RadMember for 19 years 9 months
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Suppose we have 3 tasks, A, B and C. task C should start before both A and B. If we give C a A-SF and B-SF predecessors, C will begin before later one, but I need it to start before earlier one:

Result:

A =======
B .............========
C .............^ (A-SF & B-SF)

What I want:

A =======
B .............========
C ^ (?? & ??)

(I’ve shown C with ^ because I was not sure how spaces would be shown in others’ screen. Imagine C is a milestone, but tasks would act the same).

I don’t want to remove later task’s relation, because I do not know exactly which one would start earlier, and I want a dynamic plan.

The same goes for SS predecessors. I can not make C a ALAP one, because in that case, C would push all successors in their slacks (floats).

I thought about scheduling from finish date, but it is not compatible with plan’s specifications.

Is there ***ANY*** solution for that? No matter how complicated it is.
I think there’s a serious need for SFX and SSX relations (and there would be no problem with having FSX and FFX relations too). By the way, the usual ALAP constraint will use all the Total Slack of the task; it would be nice to have a ALAPX which use only the Free Slack.

I usually use MSP, but I’m used to use Primavera besides.


Thanks
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9 replies

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  1. 0
    Brennan Westworth ·
    #1
    why not just use FS or SS with a lag?

    SF means that C should start before A and B can finnish.

    it might be easier to find a suitable solution if you tell us what the activities are.


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  2. 0
    Charleston-Joseph Orbe ·
    #2
    Nader,

    That is really a big big problem

    That is one reason i discourage to use that type of relationship.

    As much as possible use FS.

    Cheers,

    Charlie
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  3. 0
    Mel Daniels ·
    #3
    I have had similar situations occur and I completely agree with Brennan. You will be much better off with FS or SS with lag.

    I’ve had clients that would freak out by going out of sequence on logic and when I would explain to them that the activities that would normally be worked on today could not be done due to inclement weather (for example) and I decided that the better use of manpower was to have them start the other activities until weather cleared up usually did nothing for me. At least 6 out of 10 times the client would say " Why didnt you schedule it like this in the first place and hire more workers for those tasks?"

    I really hate it when they expect you to be a psychic too. lol
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  4. 0
    Nader K. Rad ·
    #4
    Thanks everybody; but using other relations would not be a solution for me.

    Suppose we have 3 tasks, each having different predecessor(s) and a start date calculated by them. Now, we need to have something prepared before starting each of them (some kind of specifications, drawings, datasheets,…). This need is going to be a milestone, and the logic behind that, is that it can simply be prepared whenever we want (doesn’t need any special work to be done). So, I want to have a milestone, showing us when this “need” should be prepared.

    My milestone should be dynamically linked to those 3 tasks, in a way that start before earliest of them.

    It is possible to make milestone FS predecessor of those three tasks, and make milestone ALAP. This works in Primavera, because Primavera uses free slack for ALAP constraint, but MSP uses total slack and this solution would not work (thanks to Paul Harris for this comment).

    I’m now looking for a solution in MSP.


    Thanks again :)
    -nader
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  5. 0
    Tahir Naseem PMP, CCE ·
    #5
    Dear Nadir
    there is sugestion, try and i hope will work for your project and not affect your other links. Creat your mile stone / activities and FF realtion with negative lag how many duration you want to for example
    activity
    A----> fs xxx
    B----> ss (10) YYY
    C----> FS A
    fs ddd
    Now D new activity developing of data sheets for civil fdn or what so ever
    D-----> FF (-duration as 30)

    Surely this will solve your problem.
    Good luck
    Tahir
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  6. 0
    Vladimir Liberzon ·
    #6
    If I understood correctly it is not a problem of SF links but the problem of "OR" logic that is not supported by Primavera and MSP.
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  7. 0
    Nader K. Rad ·
    #7
    well, I’ve found out the answer right now!
    the key is using a summary. This sample shows the solution:

    http://www.khorramirad.com/index.php?id=112

    comment: I don’t know how to paste images here, so I’m linking to my own weblog’s post about this. You can see the plan’s image there.
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  8. 0
    Tomas Rivera ·
    #8
    Nader:

    That is a nice solution.
    It solves your problem but not completely.
    You need to complete your logic and have your total float correct.
    You should add your b, c and d activities as FS succesors of your g activity.
    The way it is now, if your g activity delays, your b, c and d activities are not affected.
    I do not think this can be replicated in Primavera 5.0 since the equivalent activity type is WBS Summary which ignores relationships during scheduling.
    Any thoughts by anyone on how to replicate this in Primavera?

    Tomas Rivera
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  9. 0
    Nader K. Rad ·
    #9
    Tomas:

    Yes, you’re right, we can fix the unreal slack this way... thanks.

    mmm... I don’t have any idea about equivalent solution in Primavera...
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